Electromagnetic Radiation
(How we get most of our information about the cosmos)
Examples of electromagnetic radiation:
Light
Infrared
Ultraviolet
Microwaves
AM radio
FM radio
TV signals
Cell phone signals
X-rays
Radiation travels as waves.
Waves carry information and energy.
Properties of a wave
wavelength (l)
crest
amplitude (A)
trough
velocity (v)
l is a distance, so its units are m, cm, or mm, etc.
Also, v = l n
Period (T): time between crest (or trough) passages
Frequency (n): rate of passage of crests (or troughs), n =
(units: Hertz or cycles/sec)
1
T
 = hn
Waves
Demo: making waves - wave table
Demo: slinky waves
Radiation travels as Electromagnetic waves.
That is, waves of electric and magnetic fields traveling together.
Examples of objects with magnetic fields:
a magnet
the Earth
Clusters of galaxies
Examples of objects with electric fields:
Power lines, electric motors, …
Protons (+)
"charged" particles that
make up atoms.
Electrons (-)
}
Scottish physicist James Clerk Maxwell showed in 1865
that waves of electric and magnetic fields travel together =>
traveling “electromagnetic” waves.
The speed of all electromagnetic waves is the speed of light.
c = 3 x 10 8 m / s
or c = 3 x 10 10 cm / s
or c = 3 x 10 5 km / s
light takes 8 minutes
Earth
Sun
c= ln
or, bigger l means smaller n
The Electromagnetic Spectrum
1 nm = 10 -9 m , 1 Angstrom = 10 -10 m
c= ln
A Spectrum
Demo: white light and a prism
Refraction of light
All waves bend when they pass through materials of different densities.
When you bend light, bending angle depends on wavelength, or color.
Clicker Question:
Compared to ultraviolet radiation, infrared
radiation has greater:
A: energy
B: amplitude
C: frequency
D: wavelength
Clicker Question:
The energy of a photon is proportional to its:
A: period
B: amplitude
C: frequency
D: wavelength
Clicker Question:
A star much colder than the sun would
appear:
A: red
B: yellow
C: blue
D: smaller
E: larger
We form a "spectrum" by spreading out radiation according to
its wavelength (e.g. using a prism for light).
What does the spectrum of an astronomical object's radiation
look like?
Many objects (e.g. stars) have roughly a "Black-body"
spectrum:
• Asymmetric shape
Brightness
• Broad range of wavelengths
or frequencies
• Has a peak
Frequency
also known as the Planck spectrum or Planck curve.
Approximate black-body spectra of astronomical objects
demonstrate Wien's Law and Stefan's Law
cold dust
hotter star (Sun)
“cool" star
very hot stars
frequency increases,
wavelength decreases
Laws Associated with the Black-body Spectrum
Wien's Law:
lmax energy a
1
T
(wavelength at which most energy is radiated is longer for cooler objects)
Stefan's Law:
Energy radiated per cm2 of area on surface every second a T 4
(T = temperature at surface)
1 cm2
Betelgeuse
Rigel
Betelgeuse
The total energy radiated from entire surface every second is called the
luminosity. Thus
Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)
For a sphere, area of surface is 4pR2, where R is the sphere's radius.
The "Inverse-Square" Law Applies to Radiation
Each square gets 1/4
of the light
Each square gets 1/9
of the light
apparent brightness a 1
D2
D is the distance between
source and observer.
The Doppler Effect
Applies to all kinds of waves, not just radiation.
at rest
velocity v1
velocity v2
velocity v1
velocity v1
velocity v3
you encounter
more wavecrests
per second =>
higher frequency!
fewer wavecrests
per second =>
lower frequency!
Doppler Effect
Demo: buzzer on a moving arm
Demo: The Doppler Ball
The frequency or wavelength of a wave depends on the
relative motion of the source and the observer.
Things that waves do
1. Refraction
Waves bend when they pass through material of different densities.
air
water
swimming pool
prism
air
glass
air
2. Diffraction
Waves bend when they go through a narrow gap or around a corner.
3. Interference
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Waves can interfere with each other
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Rainbows
rred orange yellow green blue violet
What's happening in the cloud?
raindrop
42o
40o
Double Rainbows
Clicker Question:
Compared to blue light, red light travels:
A: faster in a vacuum
B: slower in a vacuum
C: at the same speed in a vacuum
Clicker Question:
Which of the following is not an
electromagnetic wave:
A: radio waves
B: visible light
C: X-rays
D: sound waves
E: gamma-rays
Clicker Question:
If a star is moving rapidly towards Earth
then its spectrum will be:
A: the same as if it were at rest
B: shifted to the blue
C: shifted to the red
D: much brighter than if it were at rest
E: much fainter than if it were at rest
Computer simulations of Interference
Radiation travels as waves.
Waves carry information and energy.
Properties of a wave
wavelength (l)
crest
amplitude (A)
trough
velocity (v)
l is a distance, so its units are m, cm, or mm, etc.
Also, v = l n
Period (T): time between crest (or trough) passages
Frequency (n): rate of passage of crests (or troughs), n =
(units: Hertz or cycles/sec)
1
T
 = hn